Water-resistant telecommunication cable

ABSTRACT

Telecommunication cable having an elongated element housing at least one transmitting element. The elongated element has a water-soluble polymeric composition of a vinyl alcohol/vinyl acetate copolymer having a saponification degree of about 60% to about 95%; a plasticizer; a hydrolysis stabilizer compound having a chelant group having two hydrogen atoms bonded to two respective heteroatoms selected from nitrogen, oxygen and sulfur. The two hydrogen atoms have a distance between each other of 4.2×10 −10  m to 5.8×10 −10  m. The stabilizer compound is present in an amount of at least 0.75 mmoles per 100 g of copolymer. The elongated element is in particular a buffer tube housing a plurality of optical fibers. The presence of the stabilizer reduces the increase of the hydrolysis degree of the copolymer upon aging, thus maintaining the desired water blocking properties of the copolymer.

FIELD OF THE INVENTION

The present invention relates to telecommunication cables, in particularoptical fiber cables, comprising elongated elements, in particularbuffer tubes, which are capable of blocking a flow of water accidentallypenetrated therein.

BACKGROUND ART

International patent application WO 00/21098, in the name of the sameApplicant and herein incorporated by reference, discloses elongatedsolid elements housing at least one optical fiber therein, said elementsbeing made from a water soluble material which, upon being contacted bywater, dissolves at least in part and forms a viscous solution ofsuitable viscosity capable of stopping the longitudinal flow of wateralong said element. The use of buffer tubes of this kind allows to avoidthe use, or at least substantially reduce the amount, of conventionalwater-blocking means, such as grease-like material, water-swellablepowders and the like.

In particular, said element is preferably a buffer tube and ispreferably made from a vinyl alcohol/vinyl acetate copolymer (VA-VAccopolymer), generally identified in the art as polyvinylalcohol. Thesecopolymers are generally obtained from partial or complete hydrolysis(i.e. saponification) of the acetate groups of a polyvinyl acetatepolymer. Thus, these materials are generally identified by theirhydrolysis (or saponification) degree, i.e. the percentage of acetategroups which has been hydrolyzed from the initial vinylacetate polymer.Typically, VA-VAc copolymer having a hydrolysis degree of 98% or higherare considered substantially completely hydrolyzed (or saponified), andare thus referred to as substantially completely hydrolyzed (orsaponified) polyvinylalcohol.

As mentioned in WO 00/21098, the water-blocking capacity of the VA-VAccopolymer depends, among other properties, also from the degree ofhydrolysis of the material. In particular VA-VAc copolymers completelyhydrolyzed are almost insoluble in water, thus being substantiallyprevented from forming the desired water-blocking viscous solution.Accordingly, WO 00/21098 suggests to employ VA-VAC copolymers having ahydolysis degree of from about 50% to 95%, preferably from 70% to about90%.

The Applicant has now observed that, while a buffer tube made from saidVA-VAC copolymer solves the problem of effectively stopping a flow ofwater accidentally penetrated inside the cable, its water blockingproperties may be impaired upon aging.

In particular the Applicant has observed that, as a consequence of theaging of the material, the water blocking properties of the material canbe impaired due to hydrolysis of the acetic groups of the copolymer. Inparticular, the degree of hydrolysis of the VA-VAc copolymer mayincrease to such an extent as to severely limit the water blockingproperties of the material.

In the art it is known to add compounds (e.g. antioxidants and/orthermal stabilizers) to polymeric compositions, to avoid oxidation andthermal degradation which may occur, for instance, at the hightemperatures during the processing of the material, e.g. during theextrusion process. These additives are however generally employed invery limited amounts. For instance, European patent EP 0 458 509discloses oxidation resistant ethylene/vinyl-alcohol copolymercompositions, having a saponification degree higher than 90%, preferablyhigher than 95% and comprising 0.01% to 0.5% w/w of a hindered phenolicantioxidant.

The Applicant has now found that the negative aging phenomena of aVA-VAC copolymer can be avoided or at least substantially reduced byadding to said copolymer an effective amount of a hydrolysis stabilizercompound having a specific chelant structure, capable of chelating thosesite on the VA-VAc copolymer chain which are deemed responsible for thehydrolysis phenomena upon aging.

SUMMARY OF THE INVENTION

A first aspect of the present invention thus relates to atelecommunication cable, in particular an optical fiber cable,comprising an elongated element housing at least one transmittingelement, said elongated element comprising a water-soluble polymericcomposition which comprises:

-   -   a vinyl alcohol/vinyl acetate copolymer having a saponification        degree of from about 60% to about 95%;    -   a plasticizer;    -   a hydrolysis stabilizer compound comprising a chelant group        comprising two hydrogen atoms bonded to two respective        heteroatoms selected from nitrogen, oxygen and sulphur?, said        two hydrogen atoms having a distance between each other of from        4.2×10⁻¹⁰ m to 5.8×10⁻¹⁰ m, preferably of from 4.5×10⁻¹⁰ m to        5.5×10⁻¹⁰ m, said stabilizer compound being present in an amount        of at least 0.75 mmoles per 100 g of VA-VAC copolymer.

Preferably the amount of said chelant group is of at least 0.8 mmoles,more preferably of at least 1.0 mmoles, per 100 g of VA-VAc copolymer.Said amount is preferably lower than about 3.5 mmoles, more preferablylower than about 3.0 mmoles, of chelant group per 100 g of VA-VAccopolymer.

Preferably, said two heteroatoms forming said chelant group are nitrogenatoms. More preferably, said two nitrogen atoms are included in tworespective amide moieties of formula —CO—NH—.

The amount of VA-VAc copolymer is preferably from about 50% to about 95%of the total weight of the polymeric composition, more preferably fromabout 60% to 85.

Preferably said plasticizer is present in an amount of from 5 to 30parts by weight per hundred parts by weight of the VA-VAc copolymer,more preferably from 10 to 25 parts.

Said stabilizer compound is preferably a compound of formula I:R—²—X¹—R¹—X²—R³  (I)

-   -   wherein    -   R¹ represents a linear or branched C₁-C₁₀ alkylene, optionally        substituted with one or two groups selected from alkyl        substituted or unsubstituted phenyl, benzyl or hydroxyphenyl;    -   X¹ and X² each independently represent a moiety comprising a        heteroatom-bonded hydrogen selected from —NH—, —CO—NH—, —CH(OH)—        or —CH(SH)—;    -   each of R² and R³ independently represent a linear or branched        C₁-C₁₀ alkyl, optionally substituted with a group selected from        alkyl substituted or unsubstituted phenyl, benzyl or        hydroxyphenyl.

In particular, the combination of groups R¹, R² and R³ is selected inorder to determine energetically feasible conformations of the molecule,wherein the distance between the heteroatom-bonded hydrogen atoms of X¹and X² is as above identified.

Preferably R² and R³ each independently represent a moiety of formula

wherein R⁴ and R⁵ independently represent a C₁—C₆ linear or branchedalkyl moiety, preferably t-butyl, and n is an integer from 0 to 6,preferably 2.

Preferably R¹ is a linear C₂-C₁₀ alkylene, more preferably a C₆alkylene.

Preferably said heteroatom moieties X₁ and X₂ are amde groups of formula—CO—NH—.

According to a particularly preferred embodiment, said stabilizercompound isN,N′-esan-1,6-dillibis[3,5-di-ter-butyl-4-hydroxyphenyl)proplonamide].

Alternatively, said stabilizer compound can be an oligomer or polymerformed by a plurality of monomeric units, each of said monomeric unitcomprising at least one heteroatom-bonded hydrogen atom, wherein theenergetically feasible conformations of the molecule provide a distancebetween two of said heteroatom-bonded hydrogen atoms of two respectivemonomeric units as above identified. For instance, said stabilizercompound can be a poll L-aminoacid of formula (III):

where n is an integer from 1 to 5.

Preferably said VA-VAc copolymer has a hydrolysis degree of from about70% to about 92%, more preferably from about 70% to about 90%.

According to a preferred embodiment, said elongated element containingthe at least one optical fiber is a tubular element comprising at leastone sheath made from said water-soluble polymeric composition.

Preferably, said tubular element comprises a double layer sheath inwhich the inner sheath is made from said water-soluble polymericcomposition and the outer sheath is made from a conventionalwater-insoluble polymer material, preferably polyethylene.

According to a further preferred embodiment, the said tubular elementcomprises a third outer sheath made of water-soluble polymericcomposition as above defined.

According to an alternative embodiment, said elongated element is agrooved core comprising at least one groove longitudinally disposed onthe outer surface of said core and housing said at least one opticalfiber. According to an embodiment of the present invention, at least thewalls of said groove are made from a water-soluble solid polymercomposition. According to an alternative embodiment, said grooved coreis made completely from said water-soluble solid polymer composition.

According to another alternative embodiment, the element made ofwater-soluble solid material included in a cable according to thepresent invention is a tape.

For the purpose of the present invention, the term “transmittingelement” includes within its meaning any element capable of transmittinga signal, particularly optical fibers, including individual opticalfibers, ribbons or bundles of optical fibers, either as such orprotected by a polymeric sheath. Non limiting examples of optical fibersare, for example, single-mode fibers, multi-mode fibers,dispersion-shifted (DS) fibers, non-zero dispersion (NZD) fibers, orfibers with a large effective area and the like, depending on theapplication requirements of the cable. They are generally fibers with anoutside diameter of between 230 and 270 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of an elongated element for a cableaccording to the invention.

FIG. 2 shows a cross-sectional view of an alternative elongated elementfor a cable according to the invention.

FIG. 3 shows a cross-sectional view of an cable according to theinvention.

FIG. 4 shows a cross-sectional view of an alternative embodiment of acable according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an example of an elongated element comprised in a cableaccording to the invention. In this embodiment, said element is atubular element in particular a buffer tube 10, which comprises apolymeric sheath 11 which envelopes a plurality of transmitting elements12. The polymeric sheath 11 is made from a water-soluble polymericmaterial as above defined. Transmitting elements are preferably opticalfibers which can be disposed inside the tube either individually, asribbons or grouped into bundles. Bundles of optical fibers (e.g. twelve)may also be semi-tightly enveloped by a so-called microsheath, i.e. asheath of polymeric material (e.g. ethylene-propylene copolymer) havinga thickness of about 0.15 mm, to form microtubes which are disposedwithin buffer tube 10. If desired, said microtubes may containwaterblocking means, in the form of grease like filler or preferably inthe form of water swellable powder compositions. For instance acomposition comprising a mixture of polyacrylate water swellableparticles and inert talc particles, as described in International PatentApplication WO 00/58768, herein incorporated by reference, can be used.If desired, some of the optical fibers housed inside said buffer tubecan be replaced by non-transmitting glass fibers, in order to reach theoptimal count within the tube, without varying the dimensions of thetube.

FIG. 2 shows a preferred embodiment of an elongated element comprised ina cable according to the invention. Buffer tube 20 comprises a doublelayer sheath, where the outermost sheath 21 is made from a conventionalpolymer material. Conventional polymer materials are, for instancepolyethylene, ethylene-propylene copolymers, polypropylene orpolybutyleneterephtalate. Preferably polyethylene, in particular highdensity polyethylene, is employed. The innermost sheath 22, enveloping aplurality of transmitting elements 12 as above defined, is made from asolid water-soluble polymer composition as described above. A buffertube 20 can be manufactured according to conventional extrusiontechniques for manufacturing double-layer sheaths, such as, forinstance, co-extrusion.

The internal diameter of buffer tubes 10 or 20 is preferably from about1.5 mm to about 2.0. The thickness of the sheath comprising thewater-soluble polymer composition is preferably from about 0.2 to about0.3 mm. The thickness of the outer polymeric layer 21 is preferably fromabout 0.2 to about 0.4 mm.

FIG. 3 shows an example of a cable according to the invention,comprising a supporting element 30, preferably made from a centralreinforcing element, typically made of glass resin, coated with a layerof polymer, e.g. polyethylene.

The cable has one or more buffer tubes 31, wound around the supportingelement 30. The buffer tubes are like those illustrated in FIG. 1 orpreferably in FIG. 2. Where appropriate, buffer tubes 31 may furthercomprise an outer layer comprising the water-blocking material.

The number of buffer tubes (which may also be arranged on severallayers) and their dimensions depend on the intended capacity of thecable, as well as on the conditions of use of this cable.

For example, cables are envisaged with only one tubular element (inwhich case the central element 30 is not present), and cables areenvisaged with six, eight or more buffer tubes, wound in one or morelayers (for example up to 24 tubular elements bundled on two layers).

The buffer tubes 31 are in turn held together by a containing layer 32,for example a wrapped polymeric tape, and are preferably surrounded by areinforcing element 33, for example a layer of Kevlar® fibres or ofglass yarn, the size of which depends on the mechanical strengthrequirements of the cable. If desired, the containing layer 32 can bemade (entirely or partly) by wrapping with a tape of water-blockingpolymer composition as above defined, or alternatively with an extrudedlayer of the same composition.

Two sheath-dividing filaments 34, arranged longitudinally with respectto the cable, can be included within the reinforcing layer 33.

The cable then comprises a protective outer sheath 35, typically made ofpolyethylene, preferably medium density polyethylene. In relation tospecific requirements, further protective layers can also be present,for example of metal layers, either inside or outside the structuredescribed.

FIG. 4 shows another embodiment of a cable according to the invention,of the slotted core type. Said cable comprises, in its radiallyinnermost position, a reinforcing element 40 made, for example, of glassresin, on which is present a grooved (or slotted) core 41 (which istypically extruded on the reinforcing element), made of water-solublepolymer composition according to the invention. The grooves 42 extend ina continuous helix or in an alternating s-z path all the way along theentire outer surface of the said core, in order to house the opticalfibres 43 therein; in a similar manner to that mentioned above, theoptical fibres can be arranged individually or assembled in ribbons,mini-tubes and the like, loosely (i.e. with an excess of length) ortightly at the bottom of the grooves.

According to an alternative embodiment, said grooved core can be madeonly in part from the water-soluble polymer composition. In this case, adouble layer grooved core is manufactured (e.g. by double-extrusion orco-extrusion of the two polymer layers), wherein the inner portion ofthe core is made from a conventional polymer material (e.g. PE or PP)and the outer grooved portion is made from a water soluble polymercomposition according to the invention.

Alternatively, the grooved core can be made entirely from a conventionalmaterial, such as PE or PP. In this case, U-shaped elongated elementsmade of water-soluble composition can be placed in its grooves, it beingpossible, for example, for these elements to be co-extruded with thegrooved core or produced separately and subsequently inserted into thegrooves.

As an example, the grooved core can be between 4 and 12 mm in diameterand can comprise from 1 to 10 grooves, depending on the capacity of thedesired cable. The dimensions of the grooves themselves are determinedby the number of fibres present therein (which may be assembled as tapesof fibres) and by the degree of freedom envisaged for these fibres.

The grooved core 41 is then coated with a layer 44 of polymer,preferably comprising said water-soluble composition, which closes offthe grooves; this coating can be made either in the form of an extrudedsheath or as a longitudinal or helical wrapping.

This layer can in turn be surrounded by a further reinforcing tape 45made, for example, of polyester, and is then surrounded by a reinforcinglayer 46 or armouring made, for example, of Kevlar®, which canincorporate filaments or rods made of solid, water-soluble composition.

A further wrapping 47 made, for example, of polyester surrounds thearmouring 46 and is in turn encircled by an outer sheath 48 typicallymade of polyethylene, particularly MDPE; a layer of water-solublepolymer composition 49 can be placed under the outer sheath 48, e.g. asa wrapped tapeor as extruded sheath, and in all of the zones which canconceivably be reached by water.

The VA-VAc copolymer comprised in an elongated element according to theinvention, is preferably selected among those having a degree ofhydrolysis of from about 60% to about 95%, more preferably from about70% to about 92%, much more preferably from about 70% to about 90%.

Furthermore, it is also preferable to use a vinyl alcohol/vinyl acetatecopolymers with a viscosity index of greater than about 10. Preferably,the viscosity index of the copolymer is between about 12 and about 40,vinyl alcohol/vinyl acetate copolymers with a viscosity index of betweenabout 15 and about 35 being particularly preferred. Advantageously, itis possible to use mixtures of copolymers with different viscosityindexes (i.e. different molecular weights), so as to combine thespecific advantageous effects of each copolymer.

Examples of commercially available materials with the desired propertiesare those sold under the trade name Mowlol® (Hoechst AG), Gohsenol®(Nippon Gohsel), Elvanol® (Du Pont) or Airvol® (Air Products).

The amount of VA-VAc copolymer in the polymeric composition forming saidelongated element is preferably from about 50% to about 95% of the totalweight of the polymeric composition, more preferably from about 60% to85.

As previously mentioned, the Applicant has found that a VA-VAc copolymerforming an elongated element according to the invention can be protectedagainst the aging effects caused by hydrolysis phenomena, by adding aneffective amount of a hydrolysis stabilizer compound comprising at leasttwo hydrogen atoms bonded to two respective heteroatoms selected fromnitrogen, oxygen and sulphur, said at least two hydrogen atoms having adistance of from 4.2×10⁻¹⁰ m to 5.8×10⁻¹⁰ m, preferably of from4.5×10⁻¹⁰ m to 5.5×10⁻¹⁰ m.

Although not willing to be bound to any particular theory, the Applicantis of the opinion that a compound with the above features is capable ofeffectively interacting with the polymeric chain of the VA-VAc copolymerin order to limit the hydrolysis of the residual acetate groups.

In particular, the distance between said hydrogen atoms in theenergetically feasible conformations (particularly in the lowest energyconformation) of the molecule of the stabilizer compound, should becapable of forming hydrogen bonds with two respective oxygen atoms ofthe VA-VAc copolymer having a distance between about 4.5×10⁻¹⁰ m to5.5×10⁻¹⁰ m.

As previously mentioned, the VA-VAc copolymer is generally obtained byhydrolysis of polyvinylacetate, by which the acetate groups of thepolymer are converted to hydroxy groups. More specifically, the reactionis typically an alcoholysis of polyvinylacetate with a metal (typicallysodium) hydroxide as catalyst. The VA-VAc copolymer resulting from thealkaline alcoholysis has mainly a block structure, where blocks formedby sequences of vinyl-acetate groups of formula —CH₂—CH(OCOCH₃)— arealternated to blocks formed by sequences of vinyl-alcohol groups offormula —CH₂—CH(OH)—.

As observed by the Applicant, upon aging and in presence of humidity,the hydrolysis reaction on a partially hydrolyzed VA-VAC copolymer tendsto continue with consequent formation of acetic acid, which in turn actsas a catalyst of the hydrolysis reaction. Due to the block structure ofthe VA-VAc copolymer, the Applicant is of the opinion that the acetategroups which are more prone to the hydrolysis attack (i.e. which requireless activation energy) are those of the terminal vinylacetate moietiesof each vinylacetate block.

The Applicant has then determined by means of structural analysis, thatthe interatomic distances between the different oxygen atoms of theacetate and hydroxy groups at the interface of the respectiveblock-terminal vinylacetate and vinylalcohol groups are comprisedbetween 4.5 and 5.4 Angstrom (10⁻¹⁰ m) in the most probableenergetically feasible conformations of the VA-VAc copolymer.

As experimentally determined by the Applicant, a chelant molecule havingtwo heteroatom-bonded hydrogen atoms at a suitable distance, i.e.comparable with those determined between the above mentioned oxygenatoms of the VA-VAc copolymer, is capable of effectively preventing thehydrolysis attack on the VA-VAc copolymer chain. It is in fact believedthat these two heteroatom-bonded hydrogen atoms are capable of forminghydrogen bonds with respective oxygen atoms on the VA-VAc copolymerchain, thus creating a chelated structure at the interface between avinylacetate and a vinylalcohol block, which effectively protects theacetate group from hydrolytic attack.

As a matter of fact, other compounds generally employed as antioxidants,which do not however show the above interatomic distance between twoheteroatom-bonded hydrogen, do not explicate the desiredaging-protective effect against hydrolysis of the acetate groups.

Determining the distance between atoms is within the skill in the art.For instance, software with which such determinations are optionallymade includes CAChe software commercially available from CACheScientific, Inc.; PCMODEL software commercially available from SerenaSoftware; HSC Chemistry for Windows, or PCMODEL software commerciallyavailable from AR Software; INSIGHT II, DISCOVER, and LUDI softwarecommercially available from BIOSYM; SYBYL, RECEPTOR, and DISCO softwarecommercially available from Tripos Associates, Inc.; and New Chem-Xcommercially available from Chemical Design, Inc. Examples of suchmodeling include those disclosed in U.S. Pat. Nos. 5,187,086; 5,250,665;4,859,769; 5,208,152; 4,980,462; 5,202,317; 5,196,404; 4,781,977, and5,175,273. Alternatively, X-ray crystallography data can be used toascertain distances. The crystallography data is preferably input to aprogrammed computer or microprocessor to determine distances.Alternatively, molecular models can be used to determine interatomicdistances when the models are dimensionally correct. Examples of suchmodeling include those disclosed in U.S. Pat. Nos. 4,877,406; 4,906,122;4,622,014, and 5,030,103. Because of speed and accuracy, determiningdistances using a programmed computer or microprocessor is preferred.

It is within the skill in the art that such distances are determined atenergetically feasible conformations, preferably the lowest energyconformation. An energetically feasible conformation is a conformationhaving a heat of formation within about 1.5 KCal (6285 Joule (J)) of theheat of formation of the lowest energy conformation. Ascertaining thelowest conformation in the energetically feasible conformation is withinthe skill in the art as shown by such references as Reviews inComputational Chemistry II, Lickowitz et al. ed., VCH Publishers, 1991,pp. 1-47 and Hehre et al., Experiments in Computational OrganicChemistry, Wavefunction, Inc., 1993, pp. 47-66. Software isadvantageously used in calculating these conformations. Software is alsoadvantageous in calculating heats of formation of each conformation andthe distances between atoms. The lowest energy conformation andenergetically feasible conformations are preferably determined by meansknown as (a) semiclassical (model), harmonic, or molecular mechanical;(b) semiempirical quantum mechanical; and/or (c) ab initio quantummechanical methods.

These methods are within the skill in the art as shown by Reviews inComputational Chemistry II, Lickowitz et al. ed., VCH Publishers, 1991,pp. 313-315.

It is also within the skilled in the art (see e.g the article from R.Scordamaglia and L. Barino “Theoretical predictive evaluation of newdonor classe in Ziegler-Natta heterogeneous catalysis for propenespecific polymerization”, Macromol. Theory simul., 7, 399-405, 1998) theuse of statistical methods applied to the molecular modelling analysis,for assigning probabilistic weights to each of the determinedenergetically feasible conformations. Upon identification of a specificmolecular parameter (such as the interatomic distance between two atomsof the molecule, as in this case), it is then possible to calculate theprobability of a selected value (or range of values) of said parameterbeing present in said molecule, as the sum of each probability assignedto the respective energetically feasible conformations showing saidselected value of said parameter.

Preferred hydrolysis stabilizer compounds are those of formula (1)previously indicated, showing the above distance between the twoheteroatom-bonded hydrogen atoms.

Examples of suitable compounds falling within the compounds defined byformula I are the following:

-   -   CH₃—CO—NH—CH₂—CH₂—C(CH₂-phenyl)₂—CH₂—CH₂—NH—CO—CH₃    -   CH₃—CH(OH)—C[(CH—(CH₃)₂]₂—]CH(OH)—CH₃    -   CH₃—NH—CH(CH₃)—C[(CH—(CH₃)_(2]) ₂—CH(CH₃)—NH—CH₃    -   Y—CH₂—CH₂—CO—NH—(CH₂)₆—NH—CO—CH₂—CH₂—Y    -   where Y is

Among those compounds of formula I, particularly preferred are thosecomprising at least one and preferably two hindered phenols, i.e. apheno, group with sterically bulky substituents located ortho to the OHmoiety. Said hindered phenols are preferably comprised in the R² and R³substituents of the compound of formula I. Examples of suitable hinderedphenols are those of formula:

-   -   wherein R⁴ and R⁵ independently represent a C₁-C₆ linear or        branched alkyl moiety, preferably t-butyl.

The presence of the hindered phenols in the stabilizer compound maycontribute to the confer (additional) thermal and oxidative stability tothe polymeric mixture during processing of the material.

Further preferred compounds are those compounds of formula I wherein theX₁ and X₂ moieties are —CO—NH— group.

A particularly preferred stabilizer compound isN,N′-esan-1,6-diilbis[3,5-di-ter-butyl-4-hydroxyphenyl)propionamide].

An example of a suitable commercially available material is Irganox 1098(Ciba Geigy).

As the effect of the stabilizer depends from the chelant group formed bythe two heteroatom-bonded hydrogen atoms, it is convenient to expressthe amount of stabilizer to be added to the VA-VAc copolymer as themillimoles of chelant groups for 100 grams of VA-VAc copolymer. In caseof a molecule of stabilizer bearing a single chelant group (such asthose compounds of formula I), the millimoles of chelant groupscorrespond to the millimoles of compound. As observed by the applicant,an amount of stbilizer of at least 0.75 mmoles per 100 g of VA-VAccopolymer, preferably of at least 0.8 mmoles, is advantageous to achievean effective stabilization of the VA-VAc copolymer. Much morepreferably, said amount is of at least 1.0 mmoles of chelant groups per100 g of VA-VAc copolymer.

The Applicant has further observed that while the amount of thehydrolysis stabilizer should be sufficiently high for resulting in thedesired stabilizing effect, it is however advisable to avoid excessiveamounts of this additives, thus keeping said amount below the aboveindicated maximum amount. As a matter of fact, excessive amounts ofstabilizer, particularly when the heteroatom groups X₁ and X₂ are aminegroups, may cause undesirable cross-linking reactions in the VA-VAccopolymer, with consequent difficulties in processing the material. Theamount of stabilizer should thus preferably be lower than about 3.5mmoles of chelant groups per 100 g of VA-VAC copolymer, more preferablylower than about 3.0 mmoles.

For instance, the Applicant has found that ifN,N′-esan-1,6-diilbis[3,5-di-ter-butyl-4-hydroxyphenyl)propionamide](Irganox 1098, Ciba Geigy) is used as stabilizer compound, it ispreferable to use from about 0.78 to about 3.2 mmoles of compound (whichcomprises a single chelant group) per 100 g of VA-VAC copolymer. Thisamount corresponds to an amount from about 0.5% to about 2.0% by weightwith respect to the total weight of VA-VAc copolymer (0.5 to 2.0 phr).

Further to the excessive amount of stabilizer, undesirable cross-linkingof the VA-VAc copolymer can also take place during the mixing of theVA-VAc copolymer with the stabilizer compound, if to much energy (heator mechanical) is transferred to the polymer during the mixing. Thestabilizer should thus preferably be admixed by controlling the energytransfer, e.g. as indicated in U.S. Pat. No. 5,137,969, hereinincorporated by reference. Preferably a co-rotating twin screw extruderis used.

The addition of a stabilizer compound as above defined allows thus toreduce the negative effects of aging on the VA-VAc copolymer, inparticular by reducing the hydrolysis phenomena of the acetate groups.The reduction of the hydrolysis phenomena determines in fact a reducedincrease in the hydrolysis degree of the material, which may thusperform its water-blocking function also after aging.

In the practice, a VA-VAc copolymer is commonly identified by means ofits saponification number, which corresponds to the mg of KOH which arenecessary to hydrolyzed one gram of VA-VAc copolymer. The hydrolysisdegree (HD) is correlated to the saponification number (SN) of theVA-VAc copolymer through the following formula:${HD} = {100 \cdot \frac{100 - {0.1535 \cdot {SN}}}{100 - {0.0749 \cdot {SN}}}}$

-   -   where the hydrolysis degree is expressed as the mole % of        hydrolyzed vinylacetate groups.

On polymeric compositions comprising a VA-VAC copolymer, it is generallyeasier to measure the saponification number of the whole composition(i.e. on the whole weight of the composition), which will thus be lowerthan the saponification number of the only VA-VAC copolymer comprised inthe composition. If necessary, by knowing the weight percentage ofVA-VAC copolymer in the compositions, it is possible to calculate thesaponification number of the VA-VAc copolymer and then, according theabove formula, the respective hydrolysis degree.

The polymeric mixture may further comprise conventional additives suchas platicizers, oxidation/thermal stabilizers, biocides, processingaids, pigments and the like.

The amount of plasticizer is preferably from about 5% to about 30% byweight with resepect to the weight of VA-VAc copolymer, more preferablyfrom about 10% to about 25%.

Examples of suitable materials which can be used as plasticizers areglycerol, sorbitol, trimethylolpropane, low molecular weight polyglycol,such as polyethylene glycol (e.g. di- or tri-ethyleneglycol),pentaerythritol, neopentylglycol, triethanolamine or oxyethylatedphosphoric esters.

Whilst the hydrolysis stabilizer of the present invention may include inthe preferred embodiment a hindered phenolic group capable of limitingthe oxidation phenomena and thermal degradation which may occur at thehigh temperatures during the processing of the material, nevertheless itmay be advantageous to add to the polymeric mixture small amounts of anoxidation/thermal stabilizer, e.g. In an amount of from about 0.05 toabout 0,5. Examples of suitable oxidation/thermal stabilizer arehindered phenolic antioxidants, such as those commercialized under thetradename Irganox by Ciba.

The following non-limitative examples are given for better illustratingthe invention.

EXAMPLES Example 1

Preparation of VA-VAC Copolymer Compositions

The hydrolysis stabilizing effect of different additives and differentamounts of said additives has been verified on a polyvynilalcoholcomposition comprising a Mowlol 26/88 (Clariant) as VA-VAc copolymer and25 phr of glycerol as plasticizer, extruded in a conventional manner toform buffer tubes.

The following additives of the hindered phenolic type have been used:

Four polymeric compositions have been prepared by adding an amount of 1phr of the above additives to the initial VA-VAC copolymer composition(i.e. 1 part of additive per 100 part by weight of VA-VAc copolymer).For Irganox 1098, this amount corresponds to 1.57 mmoles of compound per100 g of VA-VAc copolymer.

Two further comparative composition have been prepared by mixing andgranulating as above the initial VA-VAC copolymer composition, butadding to the VA-VAC copolymer and plasticizer:

-   -   0.1 phr of Irganox 245; or    -   0.1 phr of Irganox 245 and 0.32 phr of EMBAC (corresponding to        1,57 mmoles per 100 g of VA-VAc copolymer);    -   where EMBAC is the acronym indicating hexamethylenbisacetamide:    -   CH₃—CO—NH—(CH₂)₆—NH—CO—CH₃.

The six compositions are identifed as follows: Composition Additive 1Irganox 1098 2* Irganox 245 (0.1 phr) 3* Irganox 245 4* Irganox 259 5*Irganox 1010 6* EMBAC + Irganox 245*comparativeThe composition have been prepared feeding a blend with a gravimetricfeeder comprising 100 parts of Mowlol 26/88 and the additive into a 30mm co-rotating twin screw extruder (35 L/D long, vent at the 25 diameterposition) and injecting 25 parts of glycerol at the 8 diameter position.Operating conditions were as follows:

-   -   screw speed: 100 rpm    -   production rate: 10 kg/h    -   melt temperature (at the exit from the extruder: 200° C.    -   specific energy input: 0.13 KW h/kg

The strands were cooled in air and granulated into pellets.

The pellets have been subsequently extruded in the form of e buffer tube(outside diameter 2.1 mm, thickness 0.25 mm) according to conventionalextrusion techniques.

The so obtained buffer tubes were tested for measuring the number ofsaponification according to the following procedure.

Specimens of tubes of a weight about 1.0 g have are previously treatedunder a flow of 280 Nl/h of nitrogen for 1 h at the temperature of 180°C. for removing possible amounts of free acetic acid. The number ofsaponification has been determined as follows.

The specimens so treated is accurately weighed on analytical weight,inserted into a 500 ml flask, added with 100 ml of distilled water, andthe mixture is stirred under moderate heating up to dissolution of thespecimen.

25.0 ml of 0.1 N potassium hydroxide solution are then added to themixture, together with few drops of indicator, and the solution isstirred while heating to reflux for one hour.

A blank test is conducted in parallel, with the same amount ofreactants, but without the polymeric material.

Titration is effected with a 0.1 N solution of sulfuric acid.

The saponification number (i.e. the grams of reacted potassium hydroxideper gram of polymeric material) is calculated as follows:Saponification No.=5.61(PB−P)/g

where PB are the ml of sulfuric acid solution used in the blank test, Pare the ml of sulfuric acid used for the test with the polymer and g arethe grams of polymeric composition. TABLE 1 Saponification number afterextrusion Composition Saponification No. 1 112.9 2 112.3 3 113.1 4 109.65 112.1 6 108.6

As shown by the above table, no substantial variation in thesaponification number of the different compositions is observed on thenon-aged compositions.

EXAMPLE 2

Specimens of the buffer tubes obtained according to example 1 has thenbeen subjected to accelerated aging, by introducing the specimens intoan oven under controlled relative humidity (50%) at a temperature of 85°C. for 30 days.

At the end of the aging test, the saponification number of each specimenhas been measured according to the methodology described in example 1.Table 2 shows the results of the measurement, with the percentagevariation of the saponification number with respect to the one ofnon-aged specimen reported in table 1. TABLE 2 Saponification numberafter aging Saponification % variation of Composition numbersaponification no. 1 105.7 6.3 2 96.7 13.9 3 97.9 13.4 4 96.1 12.3 595.5 14.8 6 94.4 13.1

From the above table, it can be appreciated that while otherconventional hindered phenolic antioxidants are not able to limit theincrease of hydrolysis degree (i.e. the reduction of the saponificationnumber) of the VA-VAc copolymer, an effective amount of a compound asillustrated in the foregoing of the present specification substantiallylimits the hydrolytic degradation of the polymer material.

Two further test have been performed by varying the amount of Irganox1098 in a composition as previously illustrated. In particular, anamount of 0.5 phr (0.78 mmoles per 100 g of VA-VAc copolymer) and of 0.1phr have been used. In the first case, the variation of thesaponification number was of about 9.5%, while in the second case ofabout 12.1%.

1. A telecommunication cable comprising an elongated element housing atleast one transmitting element, said element comprising a water-solublepolymeric composition which comprises: a vinyl alcohol/vinyl acetatecopolymer having a saponification degree of from about 60% to about 95%;a plasticizer; and a hydrolysis stabilizer compound comprising a chelantgroup comprising two hydrogen atoms bonded to two respective heteroatomsselected from nitrogen, oxygen and sulfur, said two hydrogen atomshaving a distance between each other of 4.2×10⁻¹⁰ m to 5.8×10⁻¹⁰ m, saidstabilizer compound being present in an amount of at least 0.75 mmolesper 100 g of copolymer.
 2. The telecommunication cable according toclaim 1, wherein the amount of said chelant group is of at least 0.8mmoles per 100 g of said copolymer.
 3. The telecommunication cableaccording to claim 1, wherein the amount of said chelant group is atleast 1.0 mmoles per 100 g of said copolymer.
 4. The telecommunicationcable according to claim 1, wherein the amount of said chelant group islower than about 3.5 mmoles per 100 g of said copolymer.
 5. Thetelecommunication cable according to claim 1, wherein the amount of saidchelant group is lower than about 3.0 mmoles per 100 g of copolymer. 6.The telecommunication cable according to claim 1, wherein said twoheteroatoms forming said chelant group are nitrogen atoms.
 7. Thetelecommunication cable according to claim 6, wherein said two nitrogenatoms are included in two respective amide moieties of the formula—CO—NH—.
 8. The telecommunication cable according to claim 1, whereinthe amount of copolymer is about 50% to about 95% of the total weight ofthe polymeric composition.
 9. The telecommunication cable according toclaim 1, wherein the amount of copolymer is about 60% to 85% of thetotal weight of the polymeric composition.
 10. The telecommunicationcable according to claim 1, wherein said plasticizer is present in anamount of 5 to 30 parts by weight per hundred parts by weight of thecopolymer.
 11. The telecommunication cable according to claim 1, whereinsaid plasticizer is present in an amount of 10 to 25 parts by weight perhundred parts by weight of the copolymer.
 12. The telecommunicationcable according to claim 1, wherein said stabilizer compound is acompound of formula 1:R²—X¹—R¹—X²—R³  (I) wherein R¹ represents a linear or branched C₁-C₁₀alkylene, optionally substituted with one or two groups selected fromalkyl substituted or unsubstituted phenyl, benzyl or hydroxyphenyl; X¹and X² each independently represent a moiety comprising aheteroatom-bonded hydrogen selected from —NH—, —CO—NH—, —CH(OH)— or—CH(SH)—; and each of R² and R³ independently represent a linear orbranched C₁-C₁₀ alkyl, optionally substituted with a group selected fromalkyl substituted or unsubstituted phenyl, benzyl or hydroxyphenyl. 13.The telecommunication cable according to claim 12, wherein R² and R³each independently represent a moiety of formula (II):

wherein R⁴ and R⁵ independently represent a C₁-C₆ linear or branchedalkyl moiety, and n is an integer from 0 to
 6. 14. The telecommunicationcable according to claim 12, wherein said heteroatom moieties X₁ and X₂are amide groups of the formula —CO—NH—.
 15. The telecommunication cableaccording to claim 1, wherein said stabilizer compound isN,N′-esan-1,6-diilbis[3,5-di-(ter-butyl-4-hydroxyphenyl)propionamide].16. The telecommunication cable according to claim 1, wherein saidstabilizer compound is a poli L-aminoacid of formula (III):

where n is an integer from 1 to
 5. 17. The telecommunication cableaccording to claim 1, wherein said copolymer has a hydrolysis degree ofabout 70% to about 92%.
 18. The telecommunication cable according toclaim 1, wherein said elongated element containing at least onetransmitting element is a tubular element comprising at least one sheathmade from said water-soluble polymeric composition.
 19. Thetelecommunication cable according to claim 18, wherein said tubularelement comprises a double layer sheath in which the inner sheath ismade from said water-soluble polymeric composition and the outer sheathis made from a water-insoluble polymer material.
 20. Thetelecommunication cable according to claim 18, wherein said tubularelement further comprises a third outer sheath made from saidwater-soluble polymeric composition.
 21. The telecommunication cableaccording to claim 1, wherein said elongated element is a grooved corecomprising at least one groove longitudinally disposed on the outersurface of said core and housing said at least one transmitting element.22. The telecommunication cable according to claim 1, wherein thedistance between the two hydrogen atoms is 4.5×10⁻¹⁰ m to 5.5×10⁻¹⁰ m.23. The telecommunication cable according to claim 12, wherein thelinear or branched C₁-C₁₀ alkylene of R¹ is substituted with one or twogroups selected from alkyl, substituted or unsubstituted phenyl, benzylor hydroxyphenyl.
 24. The telecommunication cable according to claim 12,wherein the linear or branched C₁-C₁₀ alkyl of R² and R³ is substitutedwith a group selected from alkyl, substituted or unsubstituted phenyl,benzyl or hydroxyphenyl.
 25. The telecommunication cable according toclaim 13, wherein the C₁-C₆ linear or branched alkyl moiety is t-butyl.26. The telecommunication cable according to claim 13, wherein n is 2.